1. Field of the Invention
The present invention relates to immersion microscope objectives. More particularly, the present invention relates to an apochromat-class microscope objective having a magnification of about 40, a numerical aperture of about 1.0, and a flat image field.
2. Related Art
Japanese Patent Application Post-Exam Publication No. 44-16476 (1969), filed by the present assignee, discloses an objective of the same specifications as those of the present invention, which will be described later. The prior application aims at providing an objective in the apochromat class.
U.S. Pat. No. 2,644,943 discloses an objective having an arrangement which is the closest to that of the objective of the present invention among the conventional objectives. The objective has a magnification of 100 and a numerical aperture (hereinafter referred to as "NA") of 1.4, and it includes a plano-convex lens, two meniscus single lenses, a cemented lens having three lens elements, i.e., negative, positive and negative lenses, a cemented lens having two lens elements, i.e., positive and negative lenses, a cemented meniscus lens having a strong concave surface directed toward the image side, and a meniscus single lens having a strong concave surface directed toward the object side.
Japanese Patent Application Laid-Open (KOKAI) No. 58-192013 (1983), filed by the present assignee, discloses an objective having a magnification of 100 and a NA of 1.25. The objective includes a plano-convex lens, two meniscus single lenses, a cemented lens having three lens elements, i.e., positive, negative and positive lenses, a positive single lens, a cemented meniscus lens having a strong concave surface directed toward the image side, and a meniscus single lens having a strong concave surface directed toward the object side.
The objective disclosed in Japanese Patent Application Post-Exam Publication No. 44-16476 (1969) corrects chromatic aberration at the apochromat level. However, the publication makes no comment on the flatness of the image field, and the disclosed objective is extremely inferior in the flatness of the image field. The reason for this is that the objective adopts neither a buried lens for the front lens nor a Gauss type lens for the lens system.
Since the other conventional techniques and the present invention are entirely different from each other in the specifications, no direct comparison can be made between them. However, we may safely say that the prior art suffers from the following disadvantages. That is, although the objective is designed for infinity, U.S. Pat. No. 2,644,943 provides neither data on an imaging lens nor graphs showing aberrations. Therefore, the present assignee evaluated this patent by ray tracing with an imaging lens having substantially no aberration set therefor. As a result, it was revealed that spherical and comatic aberrations were exceedingly large for an objective having a NA of 1.4. The U.S. patent publication is right in stating that the Petzval sum is extremely small. However, since the coma is exceedingly large, the flatness of the image field must be said to be inferior, no matter how small the Petzval sum is. If the NA of a condenser lens in the illuminating system is reduced in actual use of the objective, the effect of coma may become inconspicuous. However, such a manner of using the objective makes the design NA of 1.4 meaningless. In view of the state of the art around the year 1950, it is considered that there was no anomalous dispersion glass material for a negative lens. Accordingly, it is expected that the objective of U.S. Pat. No. 2,644,943 suffers from large chromatic aberration (secondary spectrum), and the quality of the image observed is considerably degraded.
Japanese Patent Application Laid-Open (KOKAI) No. 58-192013 (1983) also largely differs in the specifications from the objective of the present invention, and it aims at effectively correcting various aberrations with a lens arrangement having a relatively small number of lens elements and without using a special material in a large quantity. Therefore, the present applicant will not make a comment on the conventional objective in particular.